Often in fanfiction, their relationship can change drastically. They could end up as allies, which would require both of them to change. Harry would have to see past Voldemort's evil deeds, and Voldemort would have to let go of his hatred for Harry. This exploration of character change makes fanfiction so interesting.
It might start with Voldemort's initial curiosity. He realizes Harry is more than just a threat. So he begins to observe Harry closely. As time goes by, Voldemort may start to feel a strange sense of connection, like he's the only one who truly understands Harry's power.
At first, it would probably be very tense. Harry would be suspicious of Voldemort's motives, and Voldemort would be cautious of Harry's potential to rebel. But as time goes on, they might develop a strange sort of understanding. Harry could start to see Voldemort as more than just a pure evil figure, and Voldemort might start to view Harry as something more than an enemy.
Dumbledore. His vast knowledge of magic and the wizarding world could play a crucial role. He might have some prior knowledge about the origin of the creature situation involving Harry and Voldemort. He could also offer guidance and wisdom, perhaps even try to find a way to use the situation to finally defeat Voldemort in a more permanent way.
The Weasley family in general would be involved. Ginny, for instance, would be heartbroken seeing Harry on Voldemort's side. The family's loyalty to Harry would be put to the test, and they would be among the main characters as they try to either bring Harry back or fight against him if he goes too far with Voldemort.
One possible scenario is that Harry masters a new and powerful form of magic that Voldemort has been unable to attain. For instance, if Harry discovers a way to combine different branches of magic in a unique way that gives him an edge in battles. Voldemort, who has always been obsessed with power and being the most powerful wizard, would surely be jealous of Harry's newfound abilities.
Hermione could be a main character. She might be trying to find a way to rescue Harry from the timeout situation. Ron could also be there, supporting Hermione in her efforts.
In this fanfiction, it could be that Voldemort has a premonition. He sees that if Harry Potter dies, it will lead to a series of disasters that will also destroy his own plans. So, in order to preserve the balance or to keep his long - term goals intact, he saves Harry. For example, if Harry's death causes an uncontrollable chaos in the wizarding world, Voldemort's rule over the dark forces might be disrupted. So he steps in to save Harry.
The main antagonist could be Dumbledore. Since Harry has joined Voldemort, Dumbledore would see Harry as a threat to the good of the wizarding world. Dumbledore would try to bring Harry back to the right side, and they would end up in conflict.
Sure. There are numerous fanfictions out there, and the idea of 'Harry is Voldemort's slave' could be an interesting, albeit disturbing, premise for some writers. It could be used to explore power dynamics, character development in extreme situations, and the moral implications of such a relationship. But it's important to note that this goes against the core values of the original Harry Potter story.